In a medical ultrasound system, echoes received by an ultrasound transducer are amplified by a front-end subsystem, i.e., a set of circuitry that is first to receive and process the signals provided by the transducer. As shown in FIG. 1, the front-end 100 includes a first amplifier 102, (xe2x80x9cpre-amplifierxe2x80x9d), cascaded with a variable gain amplifier 104 (xe2x80x9cVGAxe2x80x9d). The pre-amplifier 102 is coupled with a piezoelectric ultrasound transducer 106. Since the combined impedance of the piezoelectric ultrasound transducer 106 and its connecting cable (not shown) is essentially capacitive, the purpose of the pre-amplifier 102 is to convert the high impedance charge input into a usable output voltage.
Generally, two pre-amplifier 102 topologies are used. The first, known as a charge sensitive amplifier, utilizes an operational amplifier (xe2x80x9cOp Ampxe2x80x9d). FIG. 2 shows a pre-amplifier 200 which works as a charge integrator. The electrical charge from the transducer 202 is compensated by a charge of equal magnitude and opposite polarity, producing a voltage across the feedback capacitor 204, labeled xe2x80x9cCFBxe2x80x9d. In effect, the charge sensitive amplifier 200 provides a CT/CFB voltage gain, where CT is the equivalent capacitance of the transducer 202.
Another known pre-amplifier 300 architecture, shown in FIG. 3, employs a low-noise bipolar transistor 302 in a common-base configuration. Prior knowledge of the transducer 304 impedance is used to configure the pre-amplifier 300 so that the collector has an essentially capacitive load 314, labeled xe2x80x9cCLxe2x80x9d. Having approximately unity current gain, this pre-amplifier 300 causes the output charge to be equal to the input. As a result, a CT/CL voltage gain is achieved.
Referring back to FIG. 1, to compensate for the attenuation of ultrasound energy at different depths, prior art systems further utilize a variable gain-controlled amplifier 104. An exemplary VGA 104 is described in U.S. Pat. No. 4,331,929, entitled xe2x80x9cGAIN-CONTROLLED AMPLIFIERxe2x80x9d to Yokoyama. The VGA 104 adjusts the front-end gain as a function of time and/or attenuating characteristics of body tissues.
Maintaining an extensive signal range, the VGA 104 that is incorporated into a medical ultrasound system preferably provides both a low noise floor and the ability to attenuate exceptionally strong echoes while allowing progressively greater gain for echoes from deeper structures.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, one aspect of the present invention is a method for controlling input gain in a diagnostic medical ultrasound system. The method comprises receiving a plurality of ultrasound signals from a piezo-electric ultrasound transducer; varying the gain of the signals utilizing a variable capacitance; and controlling the variance of the gain as a function of the variable capacitance.
Another aspect of the present invention is a gain controlled input stage for a diagnostic medical ultrasound system. The input stage includes an ultrasound transducer, a variable capacitance circuit and an amplifier coupled with the ultrasound transducer and the variable capacitance circuit wherein the gain of the amplifier is responsive to the variable capacitance circuit.